As the state of the art, several different systems are being used to measure the speed of rotation. For instance, in DE 196 44 744 C2, the use of hall effect sensors is described. The advantages of these sensors are provided by availability at low cost, the use in a high temperature environment, and the highly accurate measurement of the speed of rotation.
In accordance with DE 10 2005 029 764 A1, an additional sensor element is represented by the MR (magnetoresistive) sensor which is suitable for registering the variations of a magnetic field. At the same rate as the above mentioned sensor units, inductive sensor elements are used for measuring the speed of rotation. They can be used as displacement sensors for determining a position of a piston, for instance known from DE 101 30 572 A1, as well as being used for measuring the speed of rotation of a part, being further explained in DE 10 2007 033 745.
As being state of the art, inductive displacement recognition comprises two or more inductors through which the calculation of the speed of rotation is accomplished by altering the frequency. In DE 101 30 572 A1, an oscillator with 8 or 16 inductors, respectively, which are connected with one or more multiplexers, a change over switch and a capacitor, the calculation is guaranteed as taking place through the frequency change in each inductor, avoiding any distortion of the measured signal. Based on the configuration in DE 101 30 572 A1, it is obvious to recognize that the parts of an inductive sensor represent a complex configuration which requires a larger effort in the circuit design for analyzing the frequency.
Another aspect of the state of the art is presented in a previous filing of DE 10 2007 033 745. Herein, the inductive sensor comprises a configuration of two inductors, both connected with oscillators and, due to their proximity to each other, are driven by a multiplexer in a way so that interaction is eliminated. The special configuration of the inductors allows a larger distance from a pulse wheel, as compared to other state of the art sensors. The larger the distance which can be realized between the pulse wheel and the inductor, preferably more than 5 to 10 mm, the easier it becomes to exchange the pulse wheel as part of the inductive sensor. However, it needs to insured that correct signals are received, despite the larger distance.